Cooperative breeding is a reproductive system in which additional group members help care for offspring beyond the care supplied by the breeding parents alone. Additional carers may be nonbreeding helpers or individuals that also share reproduction. Feeding young, defending a territory and attending a nest are different possible contributions. Group membership alone does not establish that an individual provides care.[ref]
An observational trap is that good territories may produce more surviving offspring and therefore more potential helpers. A positive association between helper number and later reproductive success need not measure the helpers' causal effect. In a Seychelles warbler study, removing additional carers and accounting for territory quality, breeder age and group composition gave a more direct comparison. One additional carer improved success in the tested circumstances; more additional adults on medium-quality territories could instead reduce it.[ref]
An original feeding example shows another subtlety. Two parents each make ten visits per hour, for twenty total. A helper contributes eight visits, while each parent reduces its contribution to six: the total remains twenty. Unchanged total provisioning would not mean the helper contributed nothing; the parents' workload changed.
The benefit to a group, the cost to a particular carer and the evolutionary explanation for helping are separate questions. A useful experiment states which outcome it measures, rather than assuming that more adults always produce more offspring or that every helper has the same relationship to the brood.
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